Typically, direct current and radio frequency discharges have been used for substrate plasma processing. More recently, plasmas for substrate processing have been produced by introducing a microwave field into a vacuum chamber containing a gas to be ionized. In conventional techniques for microwave plasma generation, the microwave field used to ionize the gas is typically not axisymmetric or tunable. However, for uniform and controllable substrate processing, it is desirable to produce a symmetric plasma from a microwave field which is axisymmetric and which can be tuned to match the impedance of the plasma load.
Presently, uniform substrate processing is typically controlled by putting the substrate on a moving turntable and shaping the microwave discharge tube to form a plasma of only a segment of the circular cross section of the microwave guide. See, for example, U.S. Pat. No. 4,430,138. By controlling the shape of this plasma, a non-axisymmetric microwave discharge can be used to process a substrate somewhat uniformly.
However, this technique exhibits a number of disadvantages, such as the fact that the microwave discharge is not truly axisymmetric and thus does not create a truly uniform plasma, and a turntable must be used to get somewhat uniform substrate processing. In addition, because these microwave plasma production devices are generally not tunable, they cannot be used for plasma production with different substrates and/or different gases at a variety of pressures. Therefore, these traditional microwave plasma production techniques have very limited fixed ranges in which they are useful.